A friend of mine has a website that has more than 135,000 unique visitors a month, and about 3.2 million page views. The site is, and always will be, free for access. Though, help is needed to pay for the high bandwidth bills.
The problem is, that he cannot use Adbrite, Yahoo and the rest, because the contents violate their TOS. Not that it's illegal or anything, just that the website is probably not suitable for everybody.
Specifics may not be necessary. The answer is fairly simple.
Evaluate your competition. Use your keywords and search on Google, Yahoo, MSN, and Ask. Join the affiliate programs they're using. Seek out industry forums (they sometimes exist) and see what affiliate programs or monetization schemes others use.
Even if there aren't webmasters at forums talking about how to monetize their niche sites, then do a search for "one of your keywords" + Forum and see how the forums are monetizing their sites.